怎么证明sinA+sinB-sinC=4sin(A/2)sin(B/2)cos(C/2)
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A,B,C是三角形的三个角吧!
右边=4sinA/2sinB/2sin（A+B）/2
=4sinA/2sinB/2[sinA/2cosB/2+sinB/2cosA/2]
=4(sinA/2)?sinB/2cosB/2+4(sinB/2）?sinA/2cosA/2
=2(sinA/2)?sinB+2(sinB/2)?sinA
=sinB(1-sinA)+sinA(1-sinB)
=sinA+sinB-sinAcosB-sinBcosA
=sinA+sinB-sin(A+B)
=sinA+sinB-sinC
=左边
右边=4sinA/2sinB/2sin（A+B）/2
=4sinA/2sinB/2[sinA/2cosB/2+sinB/2cosA/2]
=4(sinA/2)?sinB/2cosB/2+4(sinB/2）?sinA/2cosA/2
=2(sinA/2)?sinB+2(sinB/2)?sinA
=sinB(1-sinA)+sinA(1-sinB)
=sinA+sinB-sinAcosB-sinBcosA
=sinA+sinB-sin(A+B)
=sinA+sinB-sinC
=左边
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